Nitrate is the major nitrogen source for plants to uptake from soil. To meet the demands in the global supply of food, feed, fiber, and fuel, farmers tend to apply excessive nitrogen fertilizers to increase the grain yield of crops, such as maize. To avoid the pollution by nitrate and reduce the cost of farming, there is a need for plants, particularly maize, that is more efficient in nitrate uptake/utilization to maintain grain supplies and protect our environment.
Nitrate uptake from soil into the plant root cells is an active process which is against an electrochemical potential gradient of the plasma membranes. Once in the root cells, nitrate can be: 1) reduced to nitrite by the cytoplasmic enzyme nitrate reductase then ammonium by nitrite reductase in chloroplasts and then incorporate into amino acids; 2) taken up and stored in vacuole; 3) transported to the xylem for long-distance translocation into the leaves; and 4) exported out of root cells. All steps of nitrate movements are facilitated by nitrate transporters.
Nitrate transporters can be classified into two classes based on the affinity to nitrate, low- and high-affinity nitrate transporter systems. Low-affinity nitrate transporter systems (LATS) are responsible when the soil nitrate concentration is higher than 1 mM and high-affinity nitrate transporter systems (HATS) play a major role when the soil nitrate concentration is lower than 1 mM.
High-affinity nitrate transporter systems also can be classified into two groups based on if a nitrate transporter associated protein is required for nitrate transporter functionality. Single-component HATS contain a protein with typical carrier-type structure with 12 transmembrane domains and two-component HATS include an additional small associated protein with 2 transmembrance domains (Tong Y et al., Plant J., (2005) 41:442-450). Single-component HATS involved in fungi and red algae and two-component HATS have been reported in green algae and plants.
The expression of plant nitrate transporters can be constitutive or induced by nitrate. Plant nitrate transporters act as component of nitrate responsive signaling pathway and regulation of root growth independent of nitrate uptake have be reported (Little et al., PNAS (2005) 102:13693-13698). It would be desirable to improve nitrogen use efficiency and nitrate uptake of plants; however, an attempt to improve nitrate uptake by over-expressing tobacco endogenous high affinity nitrate transporters failed. (Fraisier et al., Plant J., (2000) 23:489-496).